Hello everybody. First of all, as I’m sure you’re aware, I’ve not posted a blog in quite a while. I must apologise but I’ve been incredibly busy over the past few weeks (filming left, right and centre), but I’ve had a bit of time recently so thought I’d continue with the series! Hope you guys enjoy it, be sure to leave a comment or get in touch on facebook/twitter!

Ok so this is the second blog all about colour correction, if you haven’t read the first one, here it is . If you have read the first one I hope you now have a very basic understanding of Colour Temperature. So, where do we go from here? Well there are two forms of colour correction, colour correction in camera (known as White Balance) and colour correction in lighting.

Colour correction in camera is something we’ll discuss another time, for now let’s concentrate on Colour Correction in lighting.

Daylight Colour Correction

First things first. Basically there are 3 types of lights that we use in the film and TV industry: LED, Tungsten and HMI Lighting.

LED lighting will be covered in a separate blog but for now I want you to explain the difference between Tungsten and HMI Lighting, before I go on to look at how we convert Tungsten light into daylight…

The trusty old “light bulb”!Photo Credit: Dickbauch

Tungsten Lighting

Tungsten light (or to give it a fancy name, Incandescent Light) is light produced by lamps with a filament, similar to domestic light bulbs (not energy-saving ones). The filament is made of tungsten metal… hence the term tungsten light! When electricity is passed through the bulb (known as a ‘Bubble‘ in the trade) the filament glows and emits light and heat. This light has a yellow or orange hue and when we talk about the general colour of this light we give it the term “Warm Light”.

We call it that, basically because it looks warm… not rocket science really is it?! Remember our colour temperature chart from my last blog?… well tungsten light operates around the 3,200 K range.

OK now for the second type of light…

Close up of an HMI “Bubble”Photo Credit: Daniel*D

HMI lighting

(or to give it its fancy name Hydrargyrum medium-arc iodide!.. Although I’ve never heard a Sparks call them that!).

This is what we call an arc light which means that instead of a filament (as with Incandescent light – see above) the Bulb or ‘bubble‘ is filled with a mercury vapour and has two electrodes which are separated from each other. Electricity arcs across the mercury vapour from one electrode to the other. This arcing is what produces the light. The Colour Temperature of this light is around 5,600 K range, which means that it’s equivalent to natural daylight. This light has a blue hue, and we call this “Cool Light”. Yes you’ve guest it… because it looks cold! Brrr!

HMI’s are often used to suppliment daylight on locationPhoto Credit: Eliot Lash

“So what?!”, I hear you say. Well let’s get practical, how can we use this knowledge in a real world scenario?

Real World Scenario

Say you’re going to film in a room that has daylight coming in through the windows but you need to supplement this light and the only lamps you have are Tungsten lamps. If you used these tungsten lights with the available daylight you’d end up with a strange mixture of warm (tungsten) and cold (daylight) light. We call this mixed lighting and it can actually be used to good creative effect when wanted but that’s for another time.

Good example of how “warm” tungsten light is when compared directly with daylight.Photo Credit: 663highland

So to eliminate this mixed lighting effect we need to ‘Balance’ the tungsten lights to daylight. We do this by placing special gel filters in front of the tungsten light source. These gels are semi transparent and made from heat proof polycarbonate or polyester… so don’t try using old sweet/candy wrappers, you’ll end up with a fire on your hands and bad teeth!

Gels that convert tungsten light into daylight are known as CTB (Colour Temperature Blue) gels.

Although this is a glass filter, it demonstrates the point perfectly. You can clearly see how the yellow tungsten light is being converted to blue daylight.Courtesy of Rosco Filters

I use Rosco filters all the time, they have been manufacturing gels and filters for film, TV and theatre for decades and have an enormous range. I’d highly recommend you check them out and if you need any help or advice the guys at Rosco are very approachable and helpful even if you’re just starting out.

There are various ‘grades’ of CTB that you can use, here is a Rosco Filters CTB chart which shows the range of CTB grades, click on the image to see more detail.

CTB ChartCourtesy of Rosco Filters

As you can see they range from 1/8th CTB to Full CTB. These fractions 3/4, 1/2, 1/4, and 1/8 indicate the density of the gel. For example a 1/2 CTB gel is half the strength of a (full) CTB gel.

In our example, the exact gel density you choose really depends on the colour temperature of the available daylight. At the top end of the industry, cinematographers and DP’s measure the colour temperature of the available light using a Colour Temperature Meter and then choose the correct fraction of CTB accordingly to provide a perfect colour balance. But don’t panic! Having a Colour Temperature Meter isn’t really necessary (especially for those of you who are just starting out), you can correct the light without one – as most digital cameras have monitoring/colour viewfinders it’s pretty straight forwarad to colour balance by eye. So, for the purpose of this blog I’m going to assume the colour temperature of the daylight coming into our room is 5,600K.

In this instance we would use a FULL CTBto convert the Tungsten light (3,200K) into daylight (5,600K). We would use a FULL CTB because we’re trying to balance the tungsten light with very “blue” daylight (5,600K is pretty much as blue as daylight gets. This occurs at midday). We would use the weakest density CTB filters for when the daylight is warmer, for example sunrise and sunset.

Colour Temperature changes throughout the day at sunset the colour temperature of daylight becomes lower (redder) because the sunlight is being refracted through the atmosphere in a different way compared to when it is directly overhead at midday.

One other point is that a Full CTB is quite a dense filter and will absorb quite a bit of the tungsten light source, so the more the powerful the tungten light the better.

Next time we’ll look at the reverse, converting Daylight to Tungsten.

Hope this helps explain things a bit more. Have fun!!

Remember if you have any questions about anything don’t hesitate to contact me, I’d be happy to help! I really hope you’ve enjoyed this post, please be sure to share it with your mates (it’s free!!) – I want to try and get this info to the people who want it! Oh and please show your support by following us on facebook and twitter (it’s also free!!), and if you’re interested in receiving updates and future e-books then sign up in the email form to the right.

Hey! I found your blog very intelligently presented. I have one doubt!

Q- As I learned from this blog that by adding the CTB with tungsten light, it changes to match with the daylight, but technically when we mixed blue color with yellow, then it changes to green. According to that, daylight won’t be matched with light we produced by using CTB. Please elaborate 🙂

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[…] of natural daylight is 5,600K but changes throughout the day and we’ve now talked about how to convert (or correct) tungsten light into daylight using specific blue colour temperature gels know as CTB gels. Now I want to move on and discuss […]

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My name's Chris Weaver and I know how hard it can be to break into the film/tv industry so I'm going to try and share with you information, tips, tricks & insider knowledge (as well as some bloody funny stories) from my 30 years experience as a Film & TV cameraman. Just drop by when you’ve got nothing better to do and have fun!

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